The multipath environment for a mobile wireless system can change rapidly and dramatically while a vehicle is in motion. Multiple-input multiple-output (MIMO) systems use the multipath in an environment to increase data rate by communicating over multiple parallel streams. In a typical MIMO system, the channel is estimated at the receiver, and information about the channel conditions fed back to the transmitter so that the transmitter can more effectively distribute power across the multiple parallel streams. This type of MIMO with feedback is called Informed Transmitter (IT) MIMO. If no information about the channel is available at the transmitter, the MIMO system can blindly place equal power on each of the parallel streams. This type of open-loop operation is called Uninformed Transmitter (UT) MIMO.
Many times, to reduce complexity in a realizable system, IT MIMO uses only the dominant mode of the channel to perform the MIMO processing. At low signal-to-noise ratio (SNR), dominant mode IT MIMO outperforms UT MIMO when the channel feedback is fast enough to keep up with changing channel conditions. At higher SNR, however, or when the feedback rate is too slow compared to the rate at which the channel is changing, UT MIMO outperforms dominant mode IT MIMO. A problem in practical implementations of MIMO systems that has not been addressed is adapting the operation of such a system to a channel that is changing at a rate that is too fast for IT MIMO feedback to keep up.
Studies of adaptive MIMO have considered adaptive modulation and coding based on parameters such as SINR, but have not addressed adaptive feedback methods. (See, e.g., S. Catreux, V. Erceg, D. Gesbert, and R. W. Heath, “Adaptive modulation and MIMO coding for broadband wireless data networks,” IEEE Comm. Mag., June 2002.) Heath and Paulraj proposed a method for switching between using MIMO and beamforming by measuring constellation distance, but did not consider the feedback channel. (R. W. Heath and A. Paulraj, “Switching between multiplexing and diversity based on constellation distance,” Proc. of Allerton Conf. on Comm., Control and Comp., September 2000.)
Thus, whereas known approaches have considered ways of changing modulation and other parameters based on channel conditions, there are no known approaches which consider whether or not to use feedback in the system.